The present invention relates to a process for preparing copolymers of .alpha.-methylstyrene and acrylonitrile. More particularly the present invention relates to a process for preparing such copolymers having improved uniformity of monomer content thereby achieving improved physical properties.
It is previously known in the art to prepare copolymers of .alpha.-methylstyrene and acrylonitrile. Such copolymers have desirable properties in that they exhibit improved heat resistance. Consequently, molded parts produced from such copolymers find commercial use in such applications where resistance to deformation due to the effects of elevated temperatures is particularly desired.
It is previously known in the art that copolymers of .alpha.-methylstyrene and acrylonitrile generally possess improved resistance to the effects of high heat as evidenced by a high glass transistion temperature (T.sub.g) when compared to copolymers of styrene and acrylonitrile. Processes directed to the preparation of such copolymers have been previously disclosed in U.S. Pat. Nos. 4,294,946; 4,526,928; 4,361,684; and 4,450,256; the teachings of which are herein incorporated in their entirety by reference thereto.
Despite the availability of such processes as are described in the previously disclosed United States Patents, copolymers prepared by the above identified processes have been found to be deficient in one or more physical properties. More particularly, it has been found that batch processes wherein all monomers to be polymerized in an emulsion polymerization are added initially or by means of one or two additions during the course of the polymerization result in compositions having monomer contents that vary due to the variation in monomer content experienced during the polymerization. Such variation or compositional drift is the result of a disparity in the reactivity of the two monomers, .alpha.-methylstyrene and acrylonitrile. In particular, it has been found that acrylonitrile is more reactive under standard polymerization conditions and is consumed faster than is alpha-methylstyrene monomer. Consequently, copolymers prepared from an initial charge of monomers or otherwise utilizing reaction conditions that are not designed to account for the differing reactivities of the two monomers comprise a mixture of polymers having differing monomer content.
Optimal polymer properties are believed to result from copolymers comprising .alpha.-methylstyrene and acylonitrile having a weight ratio of polymerized monomers from about 70/30 to about 85/15. Copolymeric products comprising even small amounts of polymer product of a composition outside of the above stated range tend to be deficient in desirable physical properties such as toughness or impact strength.
In the previously identified U.S. Pat. No. 4,526,928 there is disclosed a process for the preparation of copolymers of .alpha.-methylstyrene and acrylonitrile wherein the ratio of .alpha.-methylstyrene to acrylonitrile is maintained at greater than 90/10 at all times during the polymerization. While such process results in the preparation of a copolymer having a high glass transition temperature, it has now been discovered that processes involving the polymerization of monomers maintaining such a high ratio of .alpha.-methylstyrene to acrylonitrile disadvantageously are characterized by lengthy reaction times due to the excessive amount of .alpha.-methylstyrene which has a reduced reaction rate compared to acrylonitrile. Consequently, in order to provide a commerically acceptable polymerization process it would be desirable to achieve a reduction of reaction time. Moreover, it has now been discovered that glass transition temperatures of polymers prepared in the aforementioned manner are not necessarily better than those prepared utilizing reduced ratios and that the toughness of the resulting polymer (as measured by notched Izod impact values) is often inferior. This undesired result is believed to be due to compositional drift, i.e. the preparation of small amounts of polymer having inferior properties due to a gross varience in monomer ratio during the polymerization.
In U.S. Pat. No. 4,294,946 there is also disclosed a process for the preparation of copolymers of .alpha.-methylstyrene and acrylonitrile. In the disclosed process the reaction is not terminated until conversion of monomer exceeds 99%. In the process, .alpha.-methylstyrene is batched loaded and acrylonitrile is added to the reactor by a secondary addition step. Although the reference suggests that the process may be operated in a continuous manner, no teachings as to the relative rates of addition of the various monomers for such a continuous process is suggested therein. Furthermore, the reference provides no teaching as to the existence and effect of compositional drift in the copolymers or for a method for alleviating the same.
It would be desirable if there were provided an improved process for the preparing of copolymers of .alpha.-methylstyrene and acrylonitrile resulting in an improved uniformity and consequently improved physical properties thereof.